CHARLIE Gibson will become the new anchor of ABC’s “World News Tonight” on Monday – with Elizabeth Vargas moving back to “20/20” once she returns from maternity leave this fall.

The move, announced yesterday after weeks of speculation, means that Gibson will also leave his co-hosting gig on “Good Morning America” at the end of June.

“I did both [‘WNT’ and ‘GMA’] when Peter [Jennings] was sick, and I wound up with a pretty healthy case of pneumonia,” Gibson said yesterday. “It just wears you down.

“I’ll do four ‘World News Tonights’ and three ‘GMAs’ until the beginning of July, then I’ll take a little time off in July and go away and hit a golf ball or something.” Gibson’s ascension to the “WNT” anchor chair isn’t a shocker, since he was considered the front-runner for the job following Peter Jennings’ death last August – and before Vargas and Bob Woodruff were hired as co-anchors.

“David [Westin] and I had some talks about it in November and December of last year; the discussions were that David would like to have me do [‘WNT’] for a finite period of time, which eventually got to two years,” Gibson said.

Westin wanted to re-launch “WNT” with a three-anchor format of Gibson, Vargas and Woodruff, but wouldn’t give Gibson the deal he sought.

“To be honest, I really wanted to go through the 2008 election, and David said he wanted to go with the [three-anchor] experiment earlier,” Gibson said.

“David said he wanted to get on with this, and if the bottom line [for me] was going to be the election … I don’t know if I walked away or David walked away, but there were no hard feelings at all.

“We agreed to disagree, and I was happy to go back to ‘GMA,'” Gibson said. “The only leverage you have in any negotiation is the ability to walk away.” Gibson takes over a newscast that’s lost almost a million viewers from a year ago and, two weeks back, was beaten by the “CBS Evening News” for the first time in five years (but rebounded to finish second last week).

Vargas has anchored the newscast solo since Woodruff was severely injured last Jan. 29 by a roadside bomb in Iraq. He continues to recover from his injuries.

“Nobody has been through what we’ve been through, and having Charlie at the helm puts us in a heck of a position to do the broadcast we want to do,” “WNT” executive producer Jon Banner said yesterday.

“I think Charlie will bring his own sensibilities and news judgment to the program. I would expect some changes, but I wouldn’t expect them to be substantial changes.” The move also affects Woodruff.

“We don’t know,” Gibson said when asked about Woodruff’s return, and what role he might play. “It all depends on the degree to which he recovers.

You can come up with any scenario you want … but it’s going to be organic.

“This is really the ultimate exercise in patience. Everybody without exception wants Bob to come back, and he will come back,” he said.

“We’ll figure out [Woodruff’s role] when it happens. You can’t write that language into a contract.”